To produce fully industrial high frequency microwave radio systems, it is a must to make them in a Surface Mount (SMT) process. This is due to several reasons: to have as low “built-up-value” components in the final manufacturing as possible, in order to reduce cost, and to lift out chip-attach technologies and wire-bonding from “in-house-manufacturing” at radio-manufacturers, since such technologies tend to be hard to automate, which also drives cost.
There are many different types of modules for microwave radio system that may be desired to be connected to a motherboard. One example is a package which may contain some kind of microwave electronics such as a filter or a microwave integrated circuit. Another type of module may be a smaller board (e.g., a sub-board) carrying several electrical components. All such modules, however, have in common that they must be connected to the main motherboard in such a way that microwave signals can be exchanged between them efficiently.
In the prior art SMT microwave signal systems, the transferring of signals between a motherboard and a module, for instance a surface mounted package, is mostly based on connections from a microstrip to a Coplanar Waveguide to a microstrip. They work well up to around 40-50 Gigahertz (GHz) and with some limitations up to 60 GHz.
For microwave radios and automotive radar around 75-85 GHz and above another approach, Chip On Board (COB) solutions mostly are used, i.e. the chip is directly mounted on and electrically interconnected to its final circuit board, instead of first being incorporated in a package that then can be mounted on a desired board. However, the chip on board model means higher technology in the end manufacturing and such solutions are also harder and more expensive to repair.
Such Chip On Board concepts allow full SMT-manufacturing of products that can transfer microwave signals with a frequency of up to around 120 GHz.
The prior art surface mounted module systems, mentioned above, will now be described a bit more with reference to FIGS. 1 and 2. They are based on a microstrip at the motherboard and also inside the package and an inter-connection by a Coplanar Waveguide-system. In this way, the lower microstrip is lifted up to a higher microstrip. This concept gives losses and limitations when signal frequencies are passing somewhere around 40 GHz.
Such a prior art coupling arrangement 1 is shown in FIG. 1. It discloses a motherboard 2 comprising a substrate 3 and a microstrip 4. The motherboard 2 is connected to a surface mount module 5, said module comprising a substrate 6 and a micro strip conductor 7. The connection 17 between the motherboard 2 and the module 5 is shown encircled with an oval in the figure. A via-hole 18 is shown interconnecting an underside with an upper side of the substrate 6 of the module 5. In FIG. 1, X-X denotes a cross section through the connection 17; this cross section is detailed in FIG. 2.
The cross section X-X of the connection between the motherboard and the module can be studied further in FIG. 2. The motherboard 2 is connected to the module 5 via a coplanar waveguide 20. The coplanar waveguide 20 comprises two ground conductors 21 each comprising a solder pad on each of the motherboard and the module with solder in between. The ground can be seen transported from the motherboard ground plane 19 through the motherboard, by way of vias 22, to the upper side of the motherboard. The coplanar waveguide 20 further comprises, in the same plane as the ground conductors 21, a signal conductor 23 comprising the microstrip on the motherboard connected with solder to a via-hole 18 leading up to the microstrip 7 on the upper side of the module 5.
This prior art arrangement is straightforward, however the transmission of signals from microstrip to Coplanar Waveguide to microstrip is hard to maintain with a “smooth” flow at higher frequencies, which results in losses.